MX51: drop config.mk from mx51evk

The config.mk file in board directory is now obsolete and
should be removed. Add option for the IMX image into
boards.cfg

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/boards.cfg b/boards.cfg
index d4cbb06..b470bae 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -108,7 +108,7 @@
 dkb			     arm         arm926ejs   -                   Marvell        pantheon
 ca9x4_ct_vxp                 arm         armv7       vexpress            armltd
 efikamx                      arm         armv7       efikamx             -              mx5
-mx51evk                      arm         armv7       mx51evk             freescale      mx5
+mx51evk                      arm         armv7       mx51evk             freescale      mx5		mx51evk:IMX_CONFIG=board/freescale/mx51evk/imximage.cfg
 mx53evk                      arm         armv7       mx53evk             freescale      mx5
 vision2                      arm         armv7       vision2             ttcontrol      mx5
 cm_t35                       arm         armv7       cm_t35              -              omap3